Synthesis and characterization of benzilic alcohol metalloporphyrin and its nanocomposite with graphene oxide (GO–CoTHMP) and investigation of their efficiency in the removal of environmental pollutants

Abstract

5,10,15,20-Tetrakis-(4-hydroxymethylphenyl)porphyrin (THMP), metalloporphyrin [(Co(III)THMP)] and its nanocomposites with graphene oxide (GO–CoTHMP) were synthesized for the first time. The structure of synthesized GO–CoTHMP was characterized by FT-IR, UV-Vis, X-ray powder diffraction (XRD), and scanning electron microscopy (SEM) and AFM was used to assess the distances between layers in the nanocomposite. The efficiency of GO–CoTHMP in the removal of environmental contaminants was tested by degradation of methylene blue and oxidation of thiophenol, two important pollutants. The synthesized nanocomposite shows excellent selectivity in the oxidation of thiophenol and high adsorption capacity for MB and may be an effective catalyst for the removal of these environmental pollutants.

---

Introduction

Porphyrin and its derivatives are an important class of natural compounds with biological activities in the metabolism of living organisms. They have important roles in nature due to their special absorption, emission, and charge transfer and complexing properties, which are attributed to their characteristic ring structure of conjugated double bonds.¹ Some porphyrin derivatives such as metalloporphyrins can be utilized in artificial photosynthetic systems, modeling the most important function of green plants.²

Porphyrins have good absorption and can be used to reduce phenol pollutants by photooxidation processes. Tetrakis porphyrin and its metal derivatives also are strong sensitizers for this reaction.³ To understand the large variety of biological processes, including oxygen binding, electron transfer, catalysis, and the initial photochemical step in photosynthesis, knowledge of these systems and their excited states is very important. Porphyrins are in fact a main class of deeply colored pigment, of natural or synthetic origin, which share a substituted aromatic macrocycle ring comprising four pyrrole rings linked by four methine bridges.⁴

The existence of the porphyrin-like structure was first proposed by Kuster in 1912.⁵ Seventeen years later, its structure was confirmed by Hans Fischer, who successfully synthesized the haem molecule found in blood from pyrrolic starting materials.^5,6 Porphyrins have a flat structure with 22 π electrons, although only 18 of these π electrons are part of each single monocyclic delocalization route.⁶ Porphyrins can undergo additional reactions at the β–β double bond (meso position), which is more similar to alkene double bounds than aromatic ones. Moreover, 5, 10, 15, and 20 are suitable positions for electrophilic substitution reactions.

Porphyrin has a cavity in its middle, which is large enough to accommodate most metals, provided that the inner nitrogen atom is deprotonated.^4,7 These excellent properties result in a wide range of natural and biological compounds derived from porphyrins, such as haem, chlorophyll, and vitamin B₁₂.⁴ Recently, some interesting reports have been published on the application of porphyrin derivatives in the synthesis of important drugs for the therapy of diseases such as cancer, atherosclerosis, neurodegenerative diseases, and ageing.⁸ Moreover, the synthesis of new porphyrin composites, such as carbon-nanotube–porphyrin and C₆₀–porphyrin nanohybrids, has attracted great attention, and various potential applications have been explored.^9–11 Furthermore, nanohybrids combining graphene with porphyrin can be useful for a diverse range of potential applications in biology, catalysis, and sensors solar cells.¹² In addition, porphyrins are useful catalysts and are famous for their biological conduction and photoactive properties.^13–19 In recent years, the photocatalytic activities of porphyrin derivatives under visible light irradiation such as the degradation of water pollutants^20,21 has been from interesting researches in organic reactions. To enhance the photocatalytic activity of metalloporphyrins, some interesting functionalization processes using different functional groups such as COOH and NH, have been reported.²² Moreover, currently, the removal of environmental pollutants is of considerable importance with regard to green chemistry. Graphene oxide (GO), which has a wide application range, has been used to increase the efficiency of porphyrin derivatives.^23,24 In this study, we synthesized a new derivative of benzilic alcohol porphyrin and prepared its nanocomposite with GO for the first time; we then used it as a green catalyst for the optimization of pollutant removal procedures. The degradation and adsorption of methylene blue, a major environmental pollutant,^25,26 which has been shown to have very harmful effects on living things,²⁷ is an interesting application of this nanocomposite. In addition, thiols are widely distributed in petroleum products, causing foul odor and corrosion, and are considered as major environmental pollutants.¹ The oxidation of thiols to disulfides constitutes an elegant approach to remove thiols from petroleum products; however, the extracted thiols can be used in synthetic industries for various purposes.¹ Our synthesized nanocomposite was used in the oxidation of thiophenols to disulphides as a green and efficient catalyst for the removal of this pollutant. The results have shown that this oxidation was accomplished with high selectivity in the presence of this nanocatalyst.

Experimental

Materials and methods

All the chemicals used in this study were purchased from Merck and were used without further purification. ¹H-NMR spectra were obtained with a Bruker Avance 500 MHz instrument in chloroform with tetramethylsilane (Me₄Si) as the internal standard. Infrared (IR) spectra were obtained on a Shimadzu FT-IR-8400S spectrophotometer using KBr pellets. The UV-Vis spectra were obtained using a Shimadzu (mini 1240 double beam) spectrophotometer in the wavelength range of 400–800 nm. The particle morphologies were observed by scanning electron microscopy (SEM) at 26 kV (KYKY-EM3200). AFM images were obtained by DME with a Dual Scope C-21 controller and a DS 95-50 scanner. The X-ray diffraction measurements were performed using graphite monochromatic copper radiation (Cu Kα) at 40 kV, 40 mA over the 2θ range of 5–80°.

Synthesis of 5,10,15,20-tetrakis-(4-carboxymethylphenyl)porphyrin (TCMP)

Freshly distilled pyrrole (3.35 g, 3.5 mL, 50 mmol) and freshly distilled methyl 4-formylbenzoate (5.32 g, 5 mL, and 50 mmol) were refluxed in 10 mL propionic acid for 30 min following Alder's method.²⁸ The mixture was filtered when it cooled to room temperature and the solid was washed with distilled water. The resulting purple crystal was dried in air to afford TCMP (yield: 38%). IR (KBr): ν_max = 3386 (N–H), 3118 (C–H, Ar), 1722 (C=O, ester) 1606, (C=C, Ar), 1330, 1269 (=C–N), 734 cm⁻¹. UV-Vis: λ_max = 418 nm⁻¹ (Soret band), 514, 550, 590, 648 nm⁻¹ (Q bands).

Synthesis of 5,10,15,20-tetrakis-(4-hydroxymethylphenyl)porphyrin (THMP)

TCMP (3) (50 mg, 0.06 mmol) was dissolved at 0 °C in 5 mL THF. After adding LiAlH₄ (6.8 mg, 0.18 mmol), the mixture was stirred at room temperature for 3 h. Then, 1 mL of water was added to the mixture, which was stirred for 10 minutes. The solvent was removed by rotary evaporator and the solid was washed with DCM. The solid residue was washed many times with distilled water to remove traces of solvent. A brown solid was obtained and purified by column chromatography. IR (KBr): ν_max = 3417 (O–H alcohol and N–H pyrrol), 3026 (C–H, Ar), 1504 (C=C, Ar), 1353 (=C–N), 734 cm⁻¹. UV-Vis: λ_max = 414 nm⁻¹ (Soret band), 516, 552, 594, 648 nm⁻¹ (Q bands). ¹H-NMR (500 MHz, DMSO): δ in ppm = 8.79 (s, 8H CH pyrrol), 8.12–8.14 (d, 8H, J = 7.7 Hz CH Ar), 7.72–7.74 (d, 8H, J = 7.7 Hz CH Ar), 5.47 (m, 4H, OH), 4.83–4.84 (d, 8H, J = 5 Hz CH₂), −2.94 (s, 2H, N–H).

Synthesis of [5,10,15,20-tetrakis-(4-hydroxymethylphenyl)porphyrinato]cobalt(III) Cl·7H₂O (CoTHMP)

CoCl₂ (0.09 g, 6 mmol) and THMP (4) (0.09 g, 1 mmol) were dissolved in 25 mL DMSO and refluxed at 190–200 °C for 24 h according to the literature.²⁹ The product was filtered; a brown solid was obtained, purified by column chromatography and dried to afford 0.09 mg (0.089 mmol, 89%). IR (KBr): ν_max = 3450 (O–H alcohol), 1602, 1542 (C=C), 1388 (=C–N), 713, 532 cm⁻¹. UV-Vis: λ_max = 438 nm⁻¹ (Soret band), 550, 566 nm⁻¹ (Q bands).

Synthesis of [5,10,15,20-tetrakis-(4-hydroxymethylphenyl)porphyrinato]cobalt(III) Cl·7H₂O@graphene oxide (GO–CoTHMP)

Graphene oxide was synthesized according to the Hummers method.³⁰ SOCl₂ (20 mL) with DMF (0.5 mL) at 70 °C for 24 h under nitrogen atmosphere was refluxed.³⁰ At the end of the reaction, excess SOCl₂ and solvent were removed by simple distillation. In the presence of Et₃N (0.5 mL), the above product was allowed to react with CoTHMP (30 mg) in DMF (10 mL) at 130 °C for 72 h under nitrogen atmosphere. After the reaction, the solution was cooled to room temperature and then passed through filter paper. The filtrate was washed with THF several times and finally dried in an oven at 100 °C. A brown solid was obtained. IR (KBr): ν_max = 3028 (C–H, Ar), 2995 (C–H aliphatic), 1730 (C=O, ester) 1585, (C=C, Ar), 1465, (=C–N), and 1078 cm⁻¹ (C–O).

Oxidation of thiols to disulfides

Thiophenol (50 μL) and DCM (1 mL) were mixed in a round-bottomed flask. Then, CoTHMP–GO (0.005 g) was dissolved in DCM (2 mL) and added to the mixture, which was stirred for 4 h. The reaction was performed in air and the progress of the reaction was monitored by TLC. This test was performed using CoTHMP similar to above-mentioned procedure and the reaction did not proceed.

Results and discussion

The synthesis of the porphyrins, the metalloporphyrin and its nanocomposite with GO is shown in Scheme 1.

Scheme 1 Synthesis of TCMP (3), THMP (4), CoTHMP (5) and GO–CoTHMP (6).

The synthesis involved two steps to reach porphyrin (THMP) 4. Porphyrin (TCMP) 3 was synthesized according to Alder's method using distilled pyrrole and aldehyde.²⁹ We synthesized porphyrin 4 according to the study of Pasunooti and colleagues,²⁷ using LiAlH₄. Different steps were involved in the metalation of 4 with CoCL₂ in DMSO to yield (CoTHMP) 5 and finally, nanocomposite GO–CoTHMP 6 was synthesized in the presence of SoCl₂ in DMF and Et₃N.

FT-IR spectra analysis

To characterize the products, FT-IR spectra were obtained and are summarized in Fig. 1. The FT-IR spectrum of TCMP contains a sharp peak at 1722 cm⁻¹, which clearly shows that a C=O ester bond has been formed. Other important peaks appear at 3026, 1504, 1353 and 734 cm⁻¹; these can be related to C–H, C=C, =C–N and N–H in the inner cavity of porphyrin, respectively. After reduction of TCMP, the peak of the ester bond (1722 cm⁻¹) is omitted and an intense peak of THMP appears at 3417 cm⁻¹, which shows that a benzilic alcohol OH bond has been formed. After metalation of THMP with Co(II), a peak for the bending vibration of N–Co appears at 559 cm⁻¹, which indicates that CoTHMP has been synthesized. The disappearance of the peak at 3417 cm⁻¹ clearly shows that the porphyrin has been successfully metallated. The C–H stretching vibration of C=C appear at 1604 and 1560 cm⁻¹ and the =C–N of the pyrrole group appear at 1604 and 1560 cm⁻¹.

Fig. 1 FT-IR spectra of TCMP, THMP, CoTHMP and GO–CoTHMP.

As can also be observed in Fig. 2, the FT-IR spectra of GO–CoTHMP shows an intense peak at 1733 cm⁻¹, indicating that an ester bond from the reaction of the COOH groups of GO with the OH groups of the benzyl alcoholic porphyrin has been formed. Conversion of the acidic bond to an ester bond in GO–CoTHMP causes the elimination or decrease of acidic carboxylic C=O peaks in the spectra.

Fig. 2 Comparison of the FT-IR spectra of GO and GO–CoTHMP.

UV-Vis spectra analysis

The UV-Vis absorption spectra of TCMP indicate that the Soret band appears at the region of 418 nm. Four weaker absorptions, attributed to the Q bands, appear at higher wavelengths of 514, 550, 590, and 646 nm. The UV-Vis absorption spectrum of THMP in DMF (Fig. 3) indicates that the Soret band of THMP appears in the region of 414 nm. Four weaker absorptions attributed to the Q bands appear at higher wavelengths of 516, 552, 594, and 648 nm. Moreover, the spectrum of CoTHMP indicates that the Soret band of THMP appears at 419 nm. Two weaker absorptions as broad peaks, which are attributed to the Q bands, appear at 539 nm. The UV-Vis absorption spectrum of GO–CoTHMP indicates a Soret band in the region of 438 nm. Two weaker absorptions, which are related to the Q bands of the metallic complex of the alcoholic porphyrin appear at 550 and 566 nm, respectively. This higher wavelength shift can be related to the interaction of π → π* in the porphyrin and GO. Moreover, the GO peak appears at 272 nm; the appearance of the Q bands of GO–CoTHMP at 550 and 566 nm demonstrate the successful functionalization of GO with the metallic porphyrin complex.

Fig. 3 UV-Vis absorption spectra of TCMP, THMP, CoTHMP and GO–CoTHMP.

XRD studies

The XRD spectra of graphene oxide (GO) and GO–CoTHMP, shown in Fig. 4, show a characteristic diffraction peak at 11.8°. As can be observed, after immobilization of CoTHMP, the intensity of this peak decreases (due to the interaction between the GO bond and the metalloporphyrin), and another broad diffraction peak of graphene appears at a 2θ value of approximately 15° to 25°. It can be observed that the amorphization of the GO structure has increased after the functionalization process.

Fig. 4 XRD patterns of graphene oxide (GO) and functionalized graphene oxide (GO–CoTHMP).

AFM studies

Fig. 5 shows the AFM image of the synthesized nanocomposite of GO–CoTHMP. The curve of the measurement of the distance between the layers was drawn with SPM. A diluted colloidal dispersion of GO–CoTHMP was coated on mica; the AFM micrograph confirms the surface roughness and after the functionalization of GO with CoTHMP, the distance between the palate layers increased from approximately 1.5 Å to 4 Å. This average shows that complete exfoliation has occurred in our synthesized nanocomposite.

Fig. 5 AFM image of the GO–CoTHMP nanocomposite.

SEM studies

The morphology, particle size and surface uniformity of the nanocomposite can be identified by the SEM images. The image of the nanocomposite (GO–CoTHMP) in Fig. 6 shows that the average particle size is 31.4 nm. The surface of the sample is wrinkled. The image also shows that the morphology has changed and some cracks have been created due to the functionalization of graphene.

Fig. 6 SEM images of nanoparticles of (a) graphene oxide and (b) functionalized graphene oxide with CoTHMP.

Investigation of thiol oxidation

As mentioned in the introduction, thiols are widely distributed in petroleum products and are important pollutants; the oxidation of thiols to disulfides is an approach to remove thiols from petroleum products, and the extracted thiols can be used in synthetic industries for various purposes. The oxidation of thiols depends on the consumption of oxidants and produces various products such as disulfides, sulfoxides and sulfones. Disulfides are important high usage compounds, which play significant roles in biological and chemical processes and serve as versatile reagents in organic chemistry. In this study, a new nanocomposite, GO–CoTHMP, was synthesized and used as an effective catalyst in the oxidation of thiols to disulfides at room temperature and under safe conditions (Scheme 2). The oxidation of thiophenol in the presence of this versatile catalyst has been demonstrated in Table 2, in which the results clearly show that our catalyst has excellent efficiency in this reaction. Moreover, with regard for obtaining a single product, this method shows excellent selectivity in this reaction. The oxidation reaction of thiol to disulphide was performed with CoTHMP and the results showed that no oxidation occurred; moreover, oxidation in the presence of GO–CoTHMP proceeded with a high yield percentage of disulphide.

Scheme 2 Selective oxidation of thiols to disulfides.

The results in Table 1 show that the linkage of porphyrin to GO caused an elongation of the resonance system and improvement of the electron transfer process, leading to an increase in the oxidation efficiency.

Table 1 Oxidation of thiophenol in the presence of versatile catalysts³¹

	Catalyst	Time (h)	Yield
1	Pt/MgO	24 h	0%
2	Au/MgO	72 h	43%
3	Au/CeO2	30 h	13%
4	Pd/Al2O3	24 h	10%
5	Pd/MgO	24 h	23%
6	CoTHMP	4 h	Trace
7	GO–CoTHMP	4 h	91%

---

Possible mechanism of oxidation of thiophenol in the presence of GO–CoTHMP

According to numerous published studies, the oxidation of thiol to disulphide can be explained by three different reaction mechanisms. These mechanisms include electron transfer under photocatalytic conditions,^1,32,33 singlet oxygen^33,34 and molecular oxygen;^33,35–37 each of these, depending on the reaction conditions, can describe the mechanism.

Regarding our reaction conditions, the electron transfer by Co3+, the production of the RS˙ radical and finally the production of RSSR is a suitable model to describe the reaction mechanism. One important piece of evidence to confirm this mechanism is ECo3+→Co2+θ = 1.82 (V), which shows that Co3+ is an appropriate oxidant for this reaction.32,36 Co3+ can be produced by Co2+ and H2O2 in the reaction environment. Recently, the use of Mn3+ for the oxidation of thiols has been reported.34 With these considerations, the suggested mechanism of this oxidation has been proposed in Scheme 3. As can be observed, when thiophenol encounters metalloporphyrin, a cation radical is formed with the central metal. Then, by separating the cation radical and transferring electrons in porphyrin and the graphene oxide substrate, the resulting product (diphenyl disulfide) will be achieved.

Scheme 3 Possible mechanism of oxidation of thiophenol in the presence of GO–CoTHMP.

To explain the selectivity of this reaction (Scheme 2), one should refer to the obtained products, which are strongly dependent on the oxidant and catalyst and can include compounds such as disulfide, sulfoxide and sulphone. In general, two types of mechanisms are considered for this reaction. In the first, compounds such as NaBrO₃/[bmim]Br are obtained from oxidants, which perform the oxidation process by oxygen atom transfer; therefore, compounds, such as sulfoxides and sulfones, are the final products of the reaction.³⁸

Another mechanism includes electron transfer, by which the radical RS˙ is formed; disulphide is then obtained by the combination of two radicals. Moreover, the electron transfer process occurs by one- and two-electron mechanisms (depending on the oxidant); therefore, various final products can be obtained.³⁹ In this study, the electron transfer mechanism is probably the predominant mechanism and only disulphide is obtained as the final product. Of course, other parameters, such as pH and reaction temperature, are important environmental factors, which have important roles in the variety of obtained products.⁴⁰

Investigation of photodynamic degradation/absorption of methylene blue (MB)

To study the effects of porphyrin and metalloporphyrin and its synthesized nanocomposite on the elimination of pollutants, the influence of these derivatives on the degradation and absorption of methylene blue were investigated. The results have been summarized in Table 2.

Table 2 Comparison of the efficiency of various catalysts in the degradation/absorption of methylene blue

Entry	Catalyst	Degradation	Absorption
1	TCMP	80.61%	—
2	CoTCMP	69.92%	—
3	CoTHMP	—	70.63%
4	GO–CoTHMP	—	96.76%

---

The compared results show that TCMP (70.9%) and CoTCMP (80.6%) have acceptable degradation efficiency and do not show noticeable absorption. Moreover, CoTHMP (70.6%) and GO–CoTHMP (96.76%) have good absorption capacity but do not show substantial degradation. Moreover, the comparative results of the absorption of MB (20 ppm) solution after 2 h with CoTHMP and GO–CoTHMP clearly show that (similar to the oxidation reaction of thiols) the linkage of GO to CoTHMP results in improvement of the efficiency of the nanocomposite in the removal of pollutants.

As shown in Fig. 7 and 8, in the dark, there was no observable degradation; thus, light is necessary for the reaction. Moreover, in the absence of catalyst, there was no notable degradation. Among the different reaction times, the best results were obtained after 3 h.

Fig. 7 Degradation of methylene blue by synthesized TCMP.

Fig. 8 Degradation of methylene blue by synthesized CoTCMP.

As shown in Fig. 9 and 10, in the absence of catalyst, there was no significant degradation of MB. On the other hand, an almost complete decrease in the concentration of pollutant (MB) occurred in the presence of catalyst, which is related to the absorption process in the solution. In the absence of catalyst, there was no significant absorption; thus, the catalyst was necessary to carry out the reaction. Among the investigated times, 2 h had the best result in this reaction.

Fig. 9 Absorption of methylene blue by synthesized CoTHMP.

Fig. 10 Absorption of methylene blue by nanocomposite GO–CoTHMP.

Conclusions

5,10,15,20-Tetrakis-(4-caroxymethylphenyl)porphyrin (TCMP), 5,10,15,20-tetrakis-(4-hydroxymethylphenyl)porphyrin (THMP), metalloporphyrin [(Co(III)THMP)] and its nanocomposite with graphene oxide (GO–CoTHMP) were synthesized. The FT-IR, UV-Vis, SEM, XRD, AFM, and ¹H-NMR techniques were used to characterize the synthesized nanocomposite. The results clearly show the excellent efficiency of (GO–CoTHMP) in the removal of pollutants by the degradation of methylene blue and high selective oxidation of thiophenol as two important environmental pollutants.

Acknowledgements

The authors gratefully acknowledge partial support from the Research Council of the Iran University of Science and Technology.

Notes and references

1. D. Chauhan, P. Kumar, C. Joshi, N. Labhsetwar, S. K. Ganguly and S. L. Jain, New J. Chem., 2015, 39, 6193–6200.
2. G. Harriman, M. Bogue, P. RogersFani, M. Finegold, A. Bradley and S. Pacheco, FASEB J., 1996, 10(6), 1418.
3. M. Niskanen, M. Kuisma, O. Cramariuc, V. Golovanov, T. I. Hukka, N. Tkachenko and T. T. Rantala, Phys. Chem. Chem. Phys., 2013, 15, 17408–17418.
4. L. R. Milgrom, The colours of life: an introduction to the chemistry of porphyrins and related compounds, Oxford university press, Oxford, 1997, vol. vi, 225 pp., softcover, £49.50, ISBN 019-855380-3.
5. W. Kuster, Hoppe-Seylers Zeitschrift Fur Physiologische Chemie, 1912, vol. 82, pp. 463–483.
6. H. Fischer and J. Klarer, Justus Liebigs Ann. Chem., 1926, 448, 178–193.
7. D. Dolphin, The porphyrins Volume III, Academic Press, New York, 1978.
8. A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff, J. Org. Chem., 1967, 32, 476.
9. K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. Drug Discovery, 2004, 3, 205–214.
10. D. M. Guldi, G. A. Rahman, V. Sgobba and C. Ehli, Chem. Soc. Rev., 2006, 35, 471–487.
11. A. Wang, Y. Fang, L. Long, Y. Song, W. Yu, W. Zhao, M. P. Cifuentes, M. G. Humphrey and C. Zhang, Chem.–Eur. J., 2013, 19, 14159–14170.
12. A. Wang, Y. Fang, W. Yu, L. Long, Y. Song, W. Zhao, M. P. Cifuentes, M. G. Humphrey and C. Zhang, Chem.–Eur. J., 2014, 9, 639–648.
13. R. Martín-Rapffln, X. Fan, S. Sayalero, M. Bahramnejad, F. Cuevas and M. A. Pericàs, Chem.–Eur. J., 2011, 17, 8780–8783.
14. I. Willner and D. Mandler, Enzyme Microb. Technol., 1989, 11, 467–483.
15. J. P. Mahy, P. Battioni and D. Mansuy, J. Am. Chem. Soc., 1986, 108, 1079–1080.
16. D. Mandler and I. Willner, J. Phys. Chem., 1987, 91, 3600–3605.
17. I. Okura, N. Kaji, S. Aono, T. Kita and A. Yamada, Inorg. Chem., 1985, 24, 451–453.
18. I. Okura, S. Kusunoki and S. Aono, Bull. Chem. Soc. Jpn., 1984, 57, 1184–1188.
19. N. Himeshima and Y. Amao, Energy Fuels, 2003, 17, 1641–1644.
20. Z. Mehraban, F. Farzaneh and A. Shafiekhani, Opt. Mater., 2007, 29, 927–931.
21. W.-j. Sun, J. Li, G.-p. Yao, F.-x. Zhang and J.-L. Wang, Appl. Surf. Sci., 2011, 258, 940–945.
22. H. Ghafuri, Z. Movahedinia, R. Rahimi and H. R. E. Zand, RSC Adv., 2015, 5, 60172–60178.
23. X. Liu, L. Pan, T. Lv, T. Lu, G. Zhu, Z. Sun and C. Sun, Catal. Sci. Technol., 2011, 1, 1189–1193.
24. Y. Xu, L. Zhao, H. Bai, W. Hong, C. Li and G. Shi, J. Am. Chem. Soc., 2009, 131, 13490–13497.
25. R. Souther and T. Alspaugh, J. - Water Pollut. Control Fed., 1957, 29, 804.
26. K. Pirkanniemi and M. Sillanpää, Chemosphere, 2002, 48, 1047–1060.
27. K. K. Pasunooti, J.-L. Song, H. Chai, P. Amaladass, W.-Q. Deng and X.-W. Liu, J. Photochem. Photobiol., A, 2011, 218, 219–225.
28. J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney and A. M. Marguerettaz, J. Org. Chem., 1987, 52, 827–836.
29. T. Nakazono, A. R. Parent and K. Sakai, Chem. Commun., 2013, 49, 6325–6327.
30. W. S. Hummers Jr and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
31. A. Corma, J. Navas, T. Ródenas and M. J. Sabater, Chem.–Eur. J., 2013, 19, 17464–17471.
32. L. Menini, M. C. Pereira, A. C. Ferreira, J. D. Fabris and E. V. Gusevskaya, Appl. Catal., A, 2011, 392, 151–157.
33. B. Mandal and B. Basu, RSC Adv., 2014, 4, 13854–13881.
34. P. Kumar, G. Singh, D. Tripathi and S. L. Jain, RSC Adv., 2014, 4, 50331–50337.
35. T.-C. Fang, H.-Y. Tsai, M.-H. Luo, C.-W. Chang and K.-Y. Chen, Chin. Chem. Lett., 2013, 24, 145–148.
36. G. Singh, P. K. Khatri, S. K. Ganguly and S. L. Jain, RSC Adv., 2014, 4, 29124–29130.
37. W. M. Brouwer, P. Piet and A. German, J. Mol. Catal., 1984, 22, 297–308.
38. A. Shaabani, H. Mofakham, A. Rahmati and E. Farhangi, Iran. J. Chem. Chem. Eng., 2012, 31, 1–8.
39. E. Baciocchi, O. Lanzalunga, S. Malandrucco, M. Ioele and S. Steenken, J. Am. Chem. Soc., 1996, 118, 8973–8974.
40. M. Trujillo, B. Alvarez and R. Radi, Free Radical Res., 2015, 1–22.

---